DESCRIPTION (Adapted from the applicant's description): The purpose of this study is to evaluate the effectiveness of botulinum toxin injection (BTX) in reducing spasticity and improving function and mobility in children with spastic diplegia, a common form of cerebral palsy (CP). This study proposes to evaluate the effects of the treatment across the five domains of science relative to disability: pathophysiology, impairment, functional limitation, disability, and societal limitation. The study design is a prospective, randomized, double-masked clinical trial with a 6 month evaluation period. Forty children, ages 4-12 years, will be recruited and randomized into two groups. Group A will receive BTX. Group B will receive a placebo injection. Outcomes will be assessed in a masked evaluation at baseline, 3 weeks, 8 weeks, 12 weeks, and 24 weeks. Group B subjects will have the opportunity for BTX treatment after 24 weeks. Outcome variables are proposed to provide information in the five different domains. Change in pathophysiology will be represented by quantitative electromyographic kinesiology measurements. Change in impairment will be represented by electromechanical measurement of joint torque across the ankle joint using the Spasticity Measurement System. Change in functional limitation will be represented by changes in gross motor function as measured by the Gross Motor Function Measure, physical exam parameters, energy expenditure using the Energy Cost Index, and kinematic gait analysis. Changes in disability will be represented by serial assessments of age appropriate task performance using the Canadian Occupational Performance Measure. Any change in societal limitation will be measured by the use of Goal Attainment Scaling in the areas of community and school activities. Sample size has been based on the power calculation necessary to demonstrate a change in the Gross Motor Function Measure and the Spasticity Measurement System. The large number of assessments required for each patient necessitate that subject enrollment be distributed over the first four and one half years of the study.